| Summary: | 博士 === 國立臺灣大學 === 物理學研究所 === 90 === Spin-dependent tunnel junctions, Co / Al-O / Co (CoFe) / NiFe, were fabricated to investigate the effect of the additional Co (CoFe) interlayer on tunneling magneto- resistance. The quality of the junction was examined with a cross-sectional image ge- nerated by high-resolution transmission electron microscopy, and an electron energy loss spectra mapping. For junctions with a Co (CoFe) interlayer in the top electrode thinner than 0.8 nm (1.0 nm), the tunneling magnetoresistance ratio increases with interlayer thickness. For junctions with a 0.8 ~ 2.0 nm Co (1.0 ~ 2.0 nm CoFe) interlayer in the top electrode, the tunneling magnetoresistance ratio reaches the maxi- mum value of 2.16 (4.45) times that without Co (CoFe) interlayer in the top electrode. The increase in the tunneling magnetoresistance ratio may be attributed to the increa- sed effective ferromagnetic electrode polarization and the various spin-flip scattering factors.
On the other hand, the thermal stability of the spin-dependent tunnel junctions were investigated. And we focused on the roughness, thickness and diffusion behavior among ferromagnetic layers and the Al-O insulating layer after annealing. The tun- neling magnetoresistance ratio, which is 16 % for the as-deposited specimen, degrad- ed with decreasing insulator thickness and increasing interfacial roughness. Moreover, the oxygen diffuses outward and the aluminum almost stays undiffused at the annea- ling temperature up to 400 oC. The quality of amorphous Al-O insulator deteriorates due to the outward diffusion of the oxygen, causing the degradation tunneling magne- toresistance ratio.
Furthermore, the evolution of tunneling magnetoresistance as function of the angle between the external field and the film plane of the spin-dependent tunnel junctions was studied. In this work, the field dependence of the tunnel resistance (or the magnetoresistance curve) was found to be strongly angle-dependent. For the fields applied in the direction nearly parallel (perpendicular) to the film plane, the field dependent tunnel resistance was found to decrease and approach to saturation (decrease with field in the low field range and increase in the high field range). On the other hand, the tunneling magnetoresistance curve for angle near 90o reveals an anomalous behavior that the highest tunnel resistance is reached before the zero field point. The roughness induced magnetostatic coupling and the shape anisotropy may the causes of this anomalous tunneling magnetoresistance curve.
A strong temperature dependence of the characteristic behavior of the interlayer exchange bias coupling was observed in a ferromagnet / nonmagnetic metal / antife- rromagnet trilayer system (NiO/Cu/NiFe). The oscillation of the interlayer exchange bias coupling was found to be thermally assisted. At low temperature, the exchange bias field decreased monotonically with the Cu spacer thickness. Increasing the tem- perature close to the Neél temperature, the interlayer exchange bias field became osci- llatory with the Cu spacer thickness. A simple picture of the temperature-dependent competition between the RKKY-like coupling and the antiferromagnetic coupling wi- thin the antiferromagnetic layer as well as the interlayer dipolar interaction is propo- sed to explain these findings.
The magnetic relaxation behaviors of the exchange bias system and the interlayer exchange bias system were first found in this study. The effect depends clearly on the temperature and on the formation of the antiferromagnetism of the pinning layer. The temperature dependence of the fitted relaxation time and the thermal driven relaxation follow several magnetic after-effects. Finally, the interlayer exchange bias system with the thin spacer also presents the relaxation behavior of the magnetization. However, the complicated affection of the sum of the exchange coupling and the RKKY coupling on the domain wall dynamics results in the complicated temperature dependent relaxation time.
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